MARCUS W. KILPATRICK 1, ROBERT R. KRAEMER 2, EDWARD J. QUIGLEY 3, JENNIFER L. MEARS 1, JEREMY M. POWERS 1, ANTHONY J. DEDEA 1,& NICHOLAS F.

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1 Journal of Sports Sciences, March 2009; 27(5): Heart rate and metabolic responses to moderate-intensity aerobic exercise: A comparison of graded walking and ungraded jogging at a constant perceived exertion MARCUS W. KILPATRICK 1, ROBERT R. KRAEMER 2, EDWARD J. QUIGLEY 3, JENNIFER L. MEARS 1, JEREMY M. POWERS 1, ANTHONY J. DEDEA 1,& NICHOLAS F. FERRER 1 1 School of Physical Education and Exercise Science, University of South Florida, Tampa, FL, 2 Department of Kinesiology and Health Studies, Southeastern Louisiana University, Hammond, LA, and 3 Motion Analysis Laboratory, Shriner s Hospital, Tampa, FL, USA Downloaded By: [University of South Florida] At: 13:32 28 February 2009 (Accepted 5 December 2008) Abstract In this study, we assessed how ungraded jogging and graded walking at the same rating of perceived exertion (RPE) affect heart rate and oxygen consumption ( _V O 2 ). Twenty untrained participants completed a treadmill test to determine peak _V O 2 (mean ¼ ml kg 71 min 71 ). Participants completed separate 30-min trials of moderate exercise (RPE of 13 on the Borg 6 20 scale) in random order on the treadmill: graded walking and ungraded jogging. Treadmill speed or grade was adjusted throughout the trial by the experimenter based on participant responses to maintain an RPE of 13. The jogging trial produced a significantly higher heart rate ( vs beats min 71 ) and _V O 2 ( vs METs) (P ) than the walking trial. Treadmill grade decreased significantly during the walking trial ( % to %; P ), but treadmill speed did not change significantly during the jogging trial ( miles h 71 to miles h 71 )(P ), in an effort to maintain constant RPE. These findings provide evidence that similar perceptions of effort during graded walking and ungraded jogging do not produce similar cardiovascular and metabolic responses. The results indicate that, for a given prescribed perceived effort, jogging provides a greater stimulus for fitness benefits and caloric expenditure. Keywords: Exertion, exercise, prescription, perception Introduction The American College of Sports Medicine (ACSM) recently released physical activity and public health recommendations for adults (Haskell et al., 2007) that served to update and clarify earlier recommendations (Pate et al., 1995). The basic recommendations for regular participation in 30 min of moderate-intensity physical activity in the 1995 and 2007 statements are similar, but the new guidelines emphasize the benefits of vigorous exercise and identify a dose response relationship between physical activity and health. The recent release of these guidelines provides a new opportunity to consider the utility of various physical activity intensities on adherence. Early recommendations for moderaterather than high-intensity participation were likely due to the view that lower intensities are perceived more positively and provide a better opportunity for adherence. Although this important issue requires additional empirical research, a limited body of research does support this position (Lee et al., 1996; Perri et al., 2002). Nevertheless, a moderateintensity exercise regimen is likely a good choice, especially for beginning exercisers. Another important consideration regarding appropriate exercise intensity is the manner in which moderate and other intensities along the intensity continuum are defined. It has been noted that the diverse methods of defining intensity and associated intensity labels have created considerable confusion (Ekkekakis, Hall, & Petruzzello, 2005). These methods include several variations of heart rate, metabolic rate, and ratings of perceived exertion (RPE). One classification of intensity provided by ACSM is an RPE in the range when utilizing Correspondence: M. W. Kilpatrick, School of Physical Education and Exercise Science, University of South Florida, 4202 East Fowler Avenue, PED 214, Tampa, FL 33620, USA. mkilpatrick@coedu.usf.edu ISSN print/issn X online Ó 2009 Taylor & Francis DOI: /

2 510 M. W. Kilpatrick et al. the Borg 15-point category scale (Borg, 1982; Pollock et al., 1998). A value of 13 on this scale corresponds to somewhat hard and this descriptor is frequently used in practice as a proxy for an exercise intensity that is moderate. Physical activity recommendations suggest moderately intense activity and a target energy expenditure of kcal per session and 1000 kcal per week (Pate et al., 1995; US Department of Health and Human Services, 1996). Included in most recommendations are the various exercise modalities that can be used to meet physical activity guidelines. Modalities most often cited are walking and jogging, but other common modes of indoor and outdoor exercise are also mentioned. However, very little research exists comparing energy expenditure for a given level of exertion across modalities (i.e. cycling, walking, stair climbing) or within modalities (i.e. graded walking vs. ungraded jogging, upright vs. recumbent cycling). The first study to investigate energy expenditure across modalities compared popular commercial fitness equipment (Moyna et al., 2001). Research participants exercised on each modality at a moderate intensity designated by an RPE of 13 (6 20 scale) for 6 min while energy expenditure was assessed. Results indicated energy expenditure varied significantly at a given level of exertion and was highest for the treadmill, with the estimated time required to expend 200 kcal on a cycle ergometer almost twice as long as that on the treadmill despite a constant RPE. Such a finding was striking and has implications for public health recommendations. Other issues related to public health not adequately addressed by the literature include longer moderate exercise of 30 min or more, which is the primary physical activity recommendation for public health. An additional deficiency in the current literature is the lack of studies within a single mode of exercise that can be used in various ways to meet physical activity recommendations, namely the treadmill. A more detailed investigation of how speeds and grades might differentially impact heart rate and metabolic responses is appropriate given that walking and jogging represent the most common indoor and outdoor exercise modalities (US Department of Health and Human Services, 1996). The present experiment was designed to fill this void. The aim of the study was to determine how heart rate and oxygen consumption ( _V O 2 ) differ during two unique bouts of treadmill exercise performed at the same moderate RPE. Moreover, the design compared graded walking and ungraded jogging at a set RPE of 13 on the Borg 15-point category scale. We wished to determine how constraining RPE might differentially impact heart rate, _V O 2, energy expenditure, and the speed and grade required to maintain an RPE of 13 throughout the 30-min exercise period. This type of design represents an important innovation because intensity self-regulation during aerobic exercise is often managed by ratings of perceived exertion (Robertson & Noble, 1997). Methods Participants and research design The participants were 20 unpaid volunteers (11 men, 9 women; mean age 25.8 years, s ¼ 7.9) recruited from the faculty, staff, and student segments of a large university in the southeastern United States. A mixed-sex population was chosen to improve generalizability, but gender was not included in the analysis because of power considerations and because research suggests that RPE is not dependent on gender (Demello, Cureton, Boineau, & Singh, 1987). The participants completed three exercise trials each separated by at least 48 h. The first trial was a maximal protocol to measure peak V _ O 2. The other two experimental trials consisted of moderate-intensity exercise on the treadmill and were completed in a randomized, counterbalanced fashion. Each trial included a warm-up that allowed for a moderate intensity equivalent to a perceived exertion of 13 on the Borg 6 20 scale to be achieved and maintained for 30 min during graded walking or ungraded jogging (Borg, 1982). An RPE of 13 was selected because it represents a level of exertion commonly associated with a moderate intensity. Treadmill grade or speed was adjusted periodically during the trial to elicit the desired RPE response. The primary dependent measures of the study were heart rate and _V O 2, while RPE served as the primary independent variable. All participants were provided with a standardized set of instructions regarding RPE that requested whole body exertion and anchored the exertion continuum to complete rest and absolute maximum effort (ACSM, 2000). Screening Each participant completed an informed consent document, a demographic questionnaire, and a health status questionnaire. Self-reported physical activity information indicated that the participants were active, but untrained. Participants classified as low risk for exercise participation according to the ACSM guidelines (ACSM, 2000) were invited to complete the study. All participants were instructed to adequately hydrate, refrain from large meals, alcohol, caffeine, and tobacco in the 3 h before testing, and come well-rested and in exercise

3 clothing (ACSM, 2000). Additionally, participants were instructed to refrain from all exercise on the days of testing and to avoid exercise on non-test days that might cause fatigue and soreness. All research procedures were approved by the local institutional review board and conducted in accordance with institutional guidelines. Maximal exercise test A progressive, multi-stage protocol was performed on a Trackmaster treadmill (Full Vision, Newton, KS) to determine peak _V O 2. The exercise protocol began at a slow walking speed (2.0 miles h 71 for women; 2.5 miles h 71 for men) and increased by 0.5 miles h 71 each minute at a constant grade of 5%. Heart rate was assessed continuously using a Polar( heart rate monitor (Polar, USA) and recorded each minute. Rating of perceived exertion was measured each minute using the Borg 6 20 scale (Borg, 1982). Blood pressure was determined by auscultation every 3 min. Expired O 2 and CO 2 were collected through an air cushion mask, analysed continuously using a Vacumetrics MiniVista( metabolic cart (Vacumetrics, Ventura, CA) programmed for 30-s sampling intervals, and recorded each minute. Data collection and assessment intervals for all test variables were conducted according to ACSM guidelines (ACSM, 2000). Peak _V O 2 was identified as the largest volume of oxygen consumed per minute during the test. Maximal exertion was verified through the observation of peak heart rate (within 10 beats min 71 of age-predicted maximum heart rate), RPE (18 or above on the 6 20 scale), and respiratory exchange ratio (greater than 1.10). The test was terminated volitionally by the participant at the point of fatigue. Experimental exercise trials The objective of each experimental trial was to produce and maintain a moderate workload that elicited an RPE of 13 on the Borg 6 20 scale (Borg, 1982). The initial workload for both trials was light (2.5 miles h 71 and 0% grade for women; 3.0 miles h 71 and 0% grade for men) and was increased each minute by increasing either grade by 2 % for the walking trial or speed by 0.5 miles h 71 for the jogging trial. Selection of initial treadmill speed was based on pilot work, while workload increases were based on metabolic equations in an effort to produce adjustments that would result in similar metabolic increases for each stage across trials. The workload was increased until an RPE of 13 was achieved, which marked the beginning of the core 30-min aerobic session. The initial intensity loading was variable in length (range 5 10 min) and Responses to moderate-intensity aerobic exercise 511 served as the warm-up period for each submaximal trial. Each trial was concluded with a variable cooldown ranging from 3 to 5 min in length. The warmup and cool-down periods did not differ between trials (P ) and were not part of the core 30- min session. All workload changes were made by the experimenter based on responses provided by the participant during exercise. During the submaximal trials, overall body RPE was obtained and recorded every 3 min, whereas heart rate and _V O 2 were measured continuously and recorded every 3 min. The exercise trial was divided into 10 segments of 3 min each and represented 10% segments, which corresponded to 3, 6, 9, 12, 15, 18, 21, 24, 27, and 30 min. Reports of RPE that differed from the target of 13 were followed with immediate workload adjustment designed to return the participant s perception of effort to the desired target. Participants were informed in advance that the objective of workload adjustments was to reestablish an RPE of 13. The size of the adjustments was determined subjectively and was often small (i.e. 0.5% grade or 0.2 miles h 71 ) when the reported RPE increased or decreased no more than one unit. In contrast, the size of the adjustments was greater (i.e. 2.0% grade or 0.5 miles h 71 ) when the reported RPE changed by more than one unit. Before the graded exercise test, the participants read a standardized description of how perceived exertion was being operationalized in this study (Borg, 1982). Additionally, the perceived exertion scale was manipulated by adding moderate to the existing somewhat hard verbal descriptor corresponding to the 13 on the conventional 6 20 scale. This manipulation allowed the participants to be anchored to public health recommendations that use moderate as the intensity label for physical activity guidelines (USDHHS, 1996). Additionally, a value of 13 is widely considered to correspond to moderate intensities (Pollock et al., 1998) and has been shown to occur in the proximity of the ventilatory threshold (Demello et al., 1987). Statistical analyses Analyses of the data proceeded in five phases. The first phase included descriptive analysis of sample and graded exercise test characteristics. The second phase included a descriptive analysis of the RPE over time for both exercise trials. The third phase consisted of separate one-way repeated-measures analyses of variance (ANOVA) (time: 3, 6, 9, 12, 15, 18, 21, 24, 27, and 30 min) for treadmill speed and grade. The fourth phase involved a series of 2 (trial: walk vs. jog) 6 10 (time: 3, 6, 9, 12, 15, 18, 21, 24, 27, and 30 min) repeated-measures ANOVA on heart rate and _V O 2, where time and trial were

4 512 M. W. Kilpatrick et al. within-participant factors. These analyses allowed for inspection of main effects and interaction effects for each factor. Significant simple effects were followed when appropriate by planned contrasts and related marginal means were used to develop effect size (ES) differences (Hedges & Olkin, 1985). In the fifth phase, paired t-tests were used to compare physiological measures for both trials. Since these comparisons increase the risk for Type I error, the P-value for post hoc analyses was adjusted according to the Bonferroni correction to 0.01 (Hochberg, 1988). Results Graded exercise testing The participants were normal weight to marginally overweight (mean body mass index + s: kg m 72 for males and kg m 72 for females; body mass index range of 18.3 to 30.5 kg m 72 ) and untrained (peak _V O 2 : ml kg 71 min 71 for males and ml kg 71 min 71 for females). Data collected during the maximal test indicated that exhaustion was reached at maximal effort provided peak _V O 2 peak was achieved. Specifically, 11 of 20 participants achieved a heart rate within 10 beats min 71 of agepredicted maximum, 17 of 20 participants achieved an RPE of at least 18 (on a 6 20 scale), and 19 of 20 achieved a respiratory exchange ratio of at least Furthermore, maximal effort was achieved by all participants on at least two of the three indicators. Perceived exertion during submaximal exercise The primary experimental objective of establishing and maintaining a workload that elicited a score of 13 on the Borg 6 20 scale (Borg, 1982) was analysed to determine the success of the manipulation. Mean RPE values recorded during each 3-min interval were all within the range of 12.5 to Specifically, the overall mean for the jogging trial was (+s), with means for individual time points ranging from to Similarly, the overall mean for the walking trial was , with individual means ranging from to These data indicate that the manipulation to produce and maintain two moderately intense trials of exercise based on perceived exertion was successful. Speed and grade during submaximal exercise Significant effects of time were observed for grade during the walking trial (P ), but not for speed during the jogging trial (P ). These results indicate that grade decreased from the beginning to the end of the trial and that speed did not change significantly from the beginning to the end of the trial. Planned contrasts revealed that grade decreased significantly when comparing the first and last measurement time points (P ; ES ¼ 0.49), but speed did not decrease when comparing the first and last measurement time points (P ; ES ¼ 0.25). Collectively, these results indicate that maintenance of the prescribed RPE required significant reductions in workload during the walking trial but not the jogging trial. Heart rate responses during submaximal exercise A significant main effect was observed for time (P ), trial (P ), and the interaction between the two (P ). These results are presented graphically in Figure 1. The presence of main effects for time and trial indicate that heart rate differed between the two trials and that heart rate changed from the beginning to the end of the trials. The presence of an interaction effect indicates that change in heart rate over time for the two trials was different. Planned contrasts revealed that average heart rate was significantly higher in the jogging trial (P ; ES ¼ 0.87). Further planned contrasts revealed that heart rate increased for the jogging trial (P , ES ¼ 0.45), but not the walking trial (P , ES ¼ 0.14), when comparing the first and last measurements of heart rate. Metabolic responses during submaximal exercise A significant main effect on _V O 2 was observed for trial (P ) but not for time (P ) the interaction between these factors (P ). These results are presented in Figure 2. The presence of a main effect for trial indicates that _V O 2 differed between the two trials and the absence of a main effect for time indicates that _V O 2 did not change from the beginning to the end of exercise when the two trials were collapsed. Planned contrasts revealed that _V O 2 was significantly higher in the jogging trial (P ; ES ¼ 0.94). Physiological and external workload comparisons of submaximal trials Comparisons of physiological data across trials were made possible by collapsing all time points. This allowed for the determination of the average intensities (both cardiovascular and metabolic) associated with each trial. These intensities were then converted to expressions typically associated with exercise prescription to provide a separate and practical view of trial differences. Mean comparisons indicated that the two trials of exercise were significantly different in terms of various heart rate,

5 Responses to moderate-intensity aerobic exercise 513 Downloaded By: [University of South Florida] At: 13:32 28 February 2009 Figure 1. Heart rate response to exercise. Data represent mean + standard error for heart rate (beats min 71 ) during the graded walking and ungraded jogging trials. *Significantly different between trials (P ). Figure 2. Metabolic responses to exercise. Data represent mean + standard error for oxygen consumption (ml kg 71 min 71 ) during the graded walking and ungraded jogging trials. *Significantly different between trials (P ). _V O 2, and caloric expenditure indices (P ; ES range ¼ ). These data are presented in Table I. Discussion This experiment was designed to examine the cardiovascular and metabolic responses of two bouts of treadmill exercise performed at the same moderate perceived exertion but different speeds and grades. One trial involved graded walking and the other ungraded jogging. Speed was manipulated during the jogging trial and grade was manipulated during the inclined walking trial to maintain a value 13 on the Borg 6 20 scale. Any report other than 13 resulted in the adjustment of treadmill work to return the participant to the desired perceived exertion. The manipulation of constraining RPE was successful and allowed for the production of a target RPE during separate 30-min exercise sessions. The resulting responses indicated that the ungraded jogging trial was more physiologically demanding

6 514 M. W. Kilpatrick et al. Table I. Comparison of exercise trials. Graded walking (mean + s) Ungraded jogging (mean + s) Effect size Perceived exertion (6 20 scale) Treadmill * speed (miles h 71 ) Treadmill * grade (% gradient) Heart rate * (beats min 71 ) % Maximum * heart rate % Heart rate reserve * Oxygen * consumption (ml kg 71 min 71 ) MET level * % _V O 2max * % _V O 2 reserve * Caloric * expenditure (kcal) Caloric expenditure rate (kcal min 71 ) * Note: *Significantly different between trials (P ). (77% heart rate reserve; 65% _V O 2max ) than the graded walking trial (62% heart rate reserve; 52% _V O 2max ). Collectively, the physiological responses are within the ranges typically associated with a moderate exercise intensity (Pollock et al., 1998). In this study, we employed novel procedures with respect to the manipulation of RPE that are based on research designs used to prescribe exercise based on a target level of exertion. While the design includes elements of the estimation-production paradigm (Robertson & Noble, 1997), the primary intent of the methods was the development of production trials based around a target RPE linked to physical activity recommendations. Research to date that has utilized variations of the current methodology and has generally determined that RPE obtained during graded exercise testing can be used to prescribe exercise effectively (Dunbar et al., 1992; Eston, Davies & Williams, 1987; Glass, Knowlton, & Becque, 1992; Kang et al., 1998) and that exercise can be prescribed using various ratings of perceived exertion (Buckley, Eston & Sim, 2000; Eston, Parfitt, Campbell, & Lamb, 2000). Some of these studies involved the titration of exercise intensity during the production phase of the experiment to establish the external work necessary to achieve a particular perceived exertion. The current study was unique in that efforts to achieve a constant moderate exertion were maintained throughout the entire 30 min of the exercise trial through the titration of treadmill speed or grade. Results from the current study indicate that maintenance of a constant, moderate RPE throughout an exercise trial requires reductions in external work when walking on a grade, but not when jogging at a slow pace on a flat surface. The only other study to employ a similar design involved testing trained cyclists under conditions of varying ambient temperature to determine how RPE might impact heat storage during prolonged trials (Lee et al., 1996). This design required that the trials terminate when workloads declined beyond a predetermined level. Therefore, the design of the current study allowed for the novel observation of the effects of an RPE clamp between graded walking and ungraded jogging trials during 30 min of exercise on selection of workload over time. Results indicate that graded walking, but not ungraded jogging, requires reductions in workload to maintain a constant RPE. The results of the current study extend findings from previous research using short bouts of exercise indicating that levels of perceived exertion achieved by different exercise modalities do not necessarily produce similar cardiovascular and metabolic responses (Moyna et al., 2001). Specifically, although both trials resulted in a similar moderate RPE, the jogging trial produced heart rates and MET values that were 18 beats min 71 and 1.6 METs higher respectively than the moderate graded walking trial (observed MET level differences are consistent with MET tables that estimate values for the same speeds and grades). These differences in physiological responses result in very different caloric expenditure rates that have implications for health and fitness outcomes. That is, for a given perceived effort, jogging provides a greater stimulus for health and fitness benefits. This greater workload and stimulus provided by ungraded jogging could result in the expenditure of more than 15,000 additional kilocalories over the course of a year if current physical activity recommendations were adhered to. This difference in caloric expenditure could ultimately have a significant impact on weight management. Based on the methodology used in the current study, we cannot explain the cardiovascular and metabolic differences observed between these two modes of exercise at the same perceived exertion. However, two possible explanations for these findings are proffered. First, walking on a relatively severe grade may represent a novel form of exercise for many individuals, partly because of the geographic region from which the participants were recruited. This lack of familiarity with walking uphill may facilitate greater physical or mental fatigue. However, informal observations of fitness facilities

7 suggest that graded walking is a relatively common indoor fitness activity for those who would prefer to avoid running, although selecting a grade similar to that which was selected in the current study is less commonly practised by recreational exercisers. Second, research in human and animal models indicates that running at different speeds produces similar magnitudes of vertical force but different rates of force generation due to the duration of foot contact time (Kram, 2000). Specifically, running at higher speeds reduces foot contact time and increases rate of force generation, which represents a possible mediator of perceived exertion (Caferelli, 1982). This line of research has determined that the metabolic cost of running is inversely proportional to the duration of foot contact and directly proportional to the rate of generating muscle force. Applying these principles to comparisons of graded walking and jogging indicates the possibility that the present exercise trials generated similar magnitudes of muscular force at corresponding perceptions of exertion. However, the foot contact times were likely longer for graded walking than ungraded jogging, which resulted in comparatively lower rates of force generation and consequently lower metabolic costs for graded walking at the same perceived exertion. Limitations of the study primarily include those related to the sample. The current sample was relatively young, untrained, and borderline overweight. Therefore, the ability to generalize to the broader population is limited. Future studies could determine how well these findings extend to other populations. A similar investigation in an obese sample would provide additional clarity. It is possible that increased fat mass and associated changes in self-perceptions could impact perceived exertion in a way that is very different than the current study. An additional implication for future studies relates to the aforementioned possibility that a population more accustomed to more hilly terrain might respond differently than the current sample, thus further investigation is warranted. A final issue that limits the current findings is the possibility that a more moderate grade may have less of an impact on exertion than the more severe grade employed in this study. It is possible that some grade-related threshold is required to elicit an exertion that is associated with diminished physiological intensities. Future studies might be able to determine what that threshold is and whether a linear relationship exists between grade and exertion. This hypothesis is somewhat consistent with the walking speed threshold for differentiated RPE responsiveness. In summary, the results from the present study provide evidence that similar perceptions of effort during graded walking and ungraded jogging trials do not produce similar cardiovascular and metabolic Responses to moderate-intensity aerobic exercise 515 responses. Some aspect of walking at a severe grade impacts perceived exertion in a more pronounced way than does ungraded jogging. These data provide another reminder that perceptions and responses to exercise are quite varied and that exercise prescriptions should be customized to the individual participant. In addition, the current findings indicate that individuals who desire to meet recommendations for moderate physical activity participation should consider ungraded jogging as a more timeefficient modality than graded walking with respect to caloric expenditure. 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